Nearly all proteins, and many subunit interfaces in oligomeric proteins, are formed by assemblies of alpha helices and beta pleated sheets (secondary structures). In a brief preliminary paper we presented simple models to describe the rules that govern how these secondary structures pack together (Proc. Natl. Acad. Sci. U.S.A. 74, 4130-4134, 1977). By a detailed analysis of many protein structures we expect to 1) Validate and extend the applications of these simple models, 2) Show how they can be easily used to understand the structure of large complex protein molecules, 3) Show how unusual combinations of residues give rise to a few cases not described by our models.